1. File of the Invention
The present invention relates to a feed-forward amplifier and, more particularly, to a feed-forward amplifier having satisfactory linearity immediately after its activation.
2. Description of the Related Art
To amplify high-frequency carrier signals under low distortions, a feed-forward amplifier or the like is used since a feedback amplifier like that used for a low frequency band is inappropriate. The feed-forward amplifier eliminates distortions by extracting a distortion occurring in the linear amplifier itself and adding a compensation signal to the output of the linear amplifier for cancellation. The compensation signal is obtained by adjusting the gain and phase shift of the distortion signal.
FIG. 2 shows in block form the basic circuit structure of a feed-forward amplifier (see, e.g., p. 168 of "Digital Radio Communications" written by Masayoshi Muroya and Heiichi Yamamoto and published by Sangyo Tosho). As shown in FIG. 2, the feed-forward amplifier comprises a signal divider 31, a main amplifier 32, a delay line 33, an error extraction coupler 34, an auxiliary amplifier 35, a delay line 36 and an error elimination coupler 37.
As shown in FIG. 2, this feed-forward amplifier has two loops, i.e., an error detection loop and an error elimination loop. The error detection loop extracts an error (composed of a distortion component and noise) generated in the main amplifier (the target for distortion-compensation). The error elimination loop cancels the error by amplifying the extracted error, inverting the phase of the amplified error, and by combining the amplified error to the output of the main amplifier.
To implement the above function, the error detection loop is designed so that the transfer function of a path from the input terminal to the auxiliary amplifier 35 via the main amplifier 32 and the transfer function of a path from the input terminal to the auxiliary amplifier 35 via the delay line 33 are equal in amplitude but reverse-phased to each other. Therefore, only a distortion component generated in the main amplifier 32 is extracted and applied to the auxiliary amplifier 35. Similarly, the error elimination loop is designed so that the transfer function of a path from the main amplifier 32 to the output terminal via the delay line 36 and the transfer function of a path from the main amplifier 32 to the output terminal via the auxiliary amplifier 35 are equal in amplitude but reverse-phased to each other. Therefore, the error elimination loop eliminates the distortion and noise generated in the main amplifier 32. Hence, the feed-forward amplifier 32 in FIG. 2 can achieve satisfactory distortion characteristics.
As is apparent from the basic circuit structure shown in FIG. 2, to operate the feed-forward amplifier satisfactorily, the error detection loop and the error elimination loop need to be in equilibrium with high accuracy. However, it is not so easy to maintain the equilibrium. For example, to make a distortion compensation of -30 dB, the deviations of the amplitude and phase need to be within about .+-.0.1 dB and within about .+-.1 degree, respectively. However, it is difficult to maintain such conditions in the structure shown in FIG. 2. For example, the gain and phase shift amount of the main amplifier change between a state in which a carrier is applied and a state in which no carrier is applied. This is because the temperature of the amplifier changes due to a change in the heat produced by the amplifier that is brought about by the presence of a carrier. Thus, there have been proposed various automatic control techniques for maintaining the equilibrium of the feed-forward amplifier.
FIG. 3 is a block diagram of first prior art for maintaining the equilibrium of a feed-forward amplifier disclosed in Japanese Patent Application Laid Open No. Tokukohyou Sho 62-501603 (PCT).
As shown in FIG. 3, a first feed-forward amplifier of the prior art roughly comprises a signal divider 41, a coupler 42, a main amplifier 43, a coupler 44, a delay unit 45, a coupler 46, a coupler 47, an attenuator 48, a phase shifter 49, a delay unit 50, an adder 51, an attenuation and phase adjustment section 52, an automatic control circuit 53, an auxiliary amplifier 54 and a narrow-band pilot receiver 55. The narrow-band pilot receiver 55 includes a mixer 551 and an intermediate-frequency amplifier and logarithmic detector 552.
In the feed-forward amplifier shown in FIG. 3, a test signal, i.e., a pilot signal is inserted into an input signal path through the coupler 42. The inserted pilot signal is mixed with an input signal from one of the output terminals of the divider 41, and thereafter applied to the main amplifier 43. The amplitude of the resultant pilot signal is adjusted so as to be equal to the level of a distortion component generated in the main amplifier 43. The level of the distortion component is typically lower than a desired signal level by about 30 dB. The amplitude and delay of the input signal from the other output terminal of the divider 41 is adjusted so as to be equal to the amplitude and delay of the deteriorated output, but its phase is adjusted so as to be exactly reversed to the deteriorated output. The adder 51 cancels the input signal received from the delay unit 50 and the input signal component received from the coupler 44, and hence outputs only a distortion component.
The automatic control circuit 53 uses the pilot signal as a reference signal, which is detected by the narrow-band pilot receiver 55. To this end, the distortion-cancelled output from the coupler 46 is applied to the narrow-band pilot receiver 55 through the coupler 47 and the amplitude of the pilot signal is detected by the automatic control circuit 53. By adjusting the gain and phase of the attenuation and phase adjustment section 52 accurately using this information, both the pilot signal and the distortion generated in the main amplifier 43 can be canceled optimally.
In the above reference No. 62-501603, automatic control of the error detection loop is not described in detail and, generally, when the auxiliary amplifier has adequate output-power-to-distortion characteristics, it is not necessary to optimize the error detection loop at all times.
FIG. 4 is a block diagram for explaining a second prior art device disclosed in Japanese Patent Application Laid Open No. Tokukai Hei 1-198809.
As shown in FIG. 4, the feed-forward amplifier of the second prior art devices comprises a distortion detection loop 61, a power combiner 62, a distortion elimination loop 63, a directional coupler 64, a level detector 65, a synchronous detection circuit 66 and a control circuit 67. The distortion detection loop 61 includes a power divider 611, a main amplifier 612, a variable attenuator 613, a variable phase shifter 614, a pilot oscillator 615 and a directional coupler 616. The distortion elimination loop 63 includes a power combiner 631, a variable attenuator 632, a variable phase shifter 633, an auxiliary amplifier 634 and a directional coupler 635. The synchronous detection circuit 66 includes a mixer 661, a low-pass filter 662 and a dc amplifier 663.
When a signal is inputted, the level detector 65 detects the total output power level or the signal level of a predetermined frequency component, at the output stage of the auxiliary amplifier 634. The control circuit 67 adjusts the variable attenuator 613 and the variable phase shifter 614 so as to a minimize the detected signal level, so that the transfer characteristics of the two signal paths constituting the distortion detection loop 61 become in the equilibrium in which the transfer characteristics are equal in amplitude and reverse-phased to each other.
Next, the control circuit 67 adjusts the variable attenuator 632 and the variable phase shifter 633 so that the output level of the synchronous detection circuit 66 becomes minimum. Since the circuit is constructed so that the conditions for cancelling a pilot signal are the same as the conditions for cancelling a distortion generated in the main amplifier 612, the above control is effective, and the distortion elimination loop achieves the equilibrium in which the transfer characteristics of the two signal paths constituting the distortion elimination loop 63 are equal in amplitude and reverse-phased to each other.
Thus, in the second prior art feed-forward amplifier, the transfer functions of the error detection loop are automatically controlled so as to minimize the output of the auxiliary amplifier by detecting the output level of a carrier signal in the auxiliary amplifier.
FIG. 5 is a block diagram for explaining a third prior art device disclosed in Japanese Patent Application Laid Open No. Tokukai Hei 1-198809.
As shown in FIG. 5, the feed-forward amplifier of the third prior art device comprises a distortion detection loop 71, a power combiner 72, a distortion elimination loop 73, a directional coupler 74, a signal selector 75, a synchronous detection circuit 76, a control circuit 77, a pilot oscillator 78, a signal selector 79 and a directional coupler 80. The distortion detection loop 71 includes a power divider 711, a main amplifier 712, a variable attenuator 713, a variable phase shifter 714 and a directional coupler 715. The distortion elimination loop 73 includes a power combiner 731, a variable attenuator 732, a variable phase shifter 733, an auxiliary amplifier 734 and a directional coupler 735. The synchronous detection circuit 76 includes a mixer 761, a low-pass filter 762 and a dc amplifier 763.
In the third prior art feed-forward amplifier, the directional coupler 80 for injecting a pilot signal is inserted in the input side of the feed-forward amplifier. Further, the signal selector 75 is inserted in the input of the synchronous detection circuit 76, and the signal selector 79 is inserted in the output of the pilot oscillator 78.
When the signal selectors 75 and 79 are connected, respectively, by the solid lines as indicated, the operation of the feed-forward amplifier becomes equivalent to that of the second prior art device and its operation is also similar, and hence the distortion elimination loop 73 can also be automatically adjusted.
On the other hand, when the signal selectors 75 and 79 are connected, respectively, by the broken lines as indicated, the pilot signal injected by the directional coupler 80 is branched by the directional coupler 735, and the level of the branched pilot signal is detected by the synchronous detection circuit 76. The pilot signal detected by the circuit 76 is derived from deviations from the equal amplitude and reversed-phase relationship requirements imposed on the two signal paths of the distortion detection loop 71, and is, originally, of the same nature as a signal component which is to be completely suppressed. Thus, when the variable attenuator 713 and the variable phase shifter 714 are adjusted so that the detected level of this signal is reduced to a minimum, any residual signal component attributable to the disequilibrium of the distortion detection loop 71 can be reduced satisfactorily, and hence the optimal operating condition of the distortion detection loop 71 can be achieved.
Further, the control circuit 77 adjusts the variable attenuator 732 and the variable phase shifter 733 so as to minimize the output level of the synchronous detection circuit 76, so that the transfer characteristics of the two paths constituting the distortion elimination loop 73 achieve the equilibrium in which the transfer characteristics are equal in amplitude and reverse-phased to each other.
Thus, in the third prior art feed-forward amplifier, the transfer functions of the error detection loop are automatically controlled so as to minimize the level of the pilot signal by using the output of the auxiliary amplifier while injecting the pilot signal into the input of the feed-forward amplifier.
FIG. 6 is a block diagram for explaining a fourth prior art device disclosed in Japanese Patent Application Laid Open No. Tokukai Hei 5-235671.
As shown in FIG. 6, the fourth prior art feed-forward amplifier comprises a divider 91, a variable attenuator 92, a variable phase shifter 93, a main amplifier 94, a delay line 95, a divider/combiner 96, a variable attenuator 97, a variable phase shifter 98, an error amplifier 99, a delay line 100, a combiner 101, temperature sensors 102 and 103, analog-to-digital (A/D) converters 104 and 105, a central processing unit (CPU) 106, a nonvolatile memory 107, a memory 108, digital-to-analog (D/A) converters 109, 110, 111 and 112, and an input/output section (I/O) 113.
In FIG. 6, the circuit elements from the divider 91 to the combiner 101 constitute a distortion compensation amplifier. Further, the circuit elements from the divider 91 to the divider/combiner 96 constitute an error detection loop. One of the input signal components divided by the divider 91 is adjusted by the variable attenuator 92 and the variable phase shifter 93 in amplitude and phase, respectively, and the adjusted input signal component is amplified by the main amplifier 94 and applied to the divider/combiner 96. On the other hand, the other input signal component divided by the divider 91 is delayed via the delay line 95 so as to synchronize with the signal from the main amplifier 94, and the delayed input signal component is applied to the divider/combiner 96. The divider/combiner 96 detects from its two input signals an error component a (composed of a distortion component and a noise component) produced by the main amplifier 94, and outputs the detected error component a.
Further, the circuit elements from the divider/combiner 96 to the combiner 101 constitute an error elimination loop. The amplitude and phase of the error component provided by the divider/combiner 96 are adjusted by the variable attenuator 97 and the variable phase shifter 98, respectively, and the adjusted error component is amplified by the error amplifier 99 and applied to the combiner 101. On the other hand, the signal containing an error attributable to the amplification by the main amplifier 94 is provided by the divider/combiner 96, and delayed via the delay line 100 so as to synthesize with the signal from the error amplifier 99 and applied to the combiner 101. The combiner 101 inverts the phase of the error component provided by the error amplifier 99 and combines the inverted error component with the signal from the delay line 100, whereby a signal from which the error component has been cancelled is output.
Further, the circuit elements from the temperature sensor 102 to the I/O 113 constitute a temperature compensation circuit. Since the temperature characteristics of the distortion compensation amplifier are regulated by the main amplifier 94 and the error amplifier 99, the temperatures of the main amplifier 94 and the error amplifier 99 are measured by locating the temperature sensors 102 and 103 in the vicinity of the amplifiers 94 and 99, respectively. The temperature data obtained by the temperature sensors 102 and 103 are converted into digital data by the A/D converters 104 and 105, respectively, and the digital data are applied to the CPU 106. The nonvolatile memory 107, e.g., an EEPROM (Electrically Erasable Programmable Read-Only Memory) has values previously written respectively for the variable attenuator 92, the variable phase shifter 93, the variable attenuator 97 and the variable phase shifter 98 so that the distortion of the distortion compensation amplifier is minimized within a predicted temperature range. During the operation, the CPU 106 moves the data in the nonvolatile memory 107 to the memory 108, writes an assumed intermediate temperature between the read temperatures, reads the values in the memory 108 corresponding to the temperatures detected by the temperature sensors 102 and 103, converts the read values into analog values through the D/A converters 109, 110, 111 and 112, and applies the analog values to the variable attenuator 92, the variable phase shifter 93, the variable attenuator 97 and the variable phase shifter 98, whereby these values are controlled so as to be optimal.
Thus, the fourth prior art feed-forward amplifier aims at maintaining the transfer functions of the error detection loop and the transfer functions of the error elimination loop constant not only by locating the temperature sensors in the vicinity of the amplifiers and controlling the transfer functions of the error detection loop and the transfer functions of the error elimination loop in accordance with the temperatures measured in the vicinity of the amplifiers, respectively, but also by changing the control values while predicting the characteristics of the amplifiers which change due to a temperature change using the temperatures measured in the vicinity of the amplifiers.
However, these conventional feed-forward amplifiers have addressed the problem that their operations are not always satisfactory.
First of all, the first prior art does not describe details about the automatic control of the error detection loop. Generally, it is not necessary for the feed-forward amplifier to optimize the transfer functions of the error detection loop by providing special automatic control means, as long as the auxiliary amplifier has output power with a sufficient margin. However, in order to improve the power utilization efficiency and curtail the production cost, the output powers of the main amplifier and the auxiliary amplifier need to be reduced to a necessary minimum. An order to reduce the output powers of the amplifiers to a necessary minimum, it is preferable to employ automatic control so that the amplifiers can accommodate temperature changes and long term fluctuations. Thus, to implement a more inexpensive and more highly efficient feed-forward amplifier, automatic control is effected for the error detection loop so that the transfer functions of the error detection loop are optimally controlled.
Secondly, the technique disclosed in the second prior art device, in which the distortion detection loop (the error detection loop) is automatically controlled so as to minimize the output of the auxiliary amplifier by detecting the output level of the auxiliary amplifier in order to automatically control the error detection loop, does not allow automatic control to be effected unless a carrier is actually applied. This is because automatic control is effected by utilizing an input signal to be amplified. Thus, this technique addresses a shortcoming that distortion characteristics immediately after the activation are poor. That is, the amplifier characteristics immediately after the application of a carrier are not satisfactory.
Further, since the input signal is generally modulated, control is susceptible to errors unless the time constant of a smoothing circuit in the detector is sufficiently increased compared with the modulated signal or the like. On the other hand, when the time constant of the detector is increased, it becomes difficult to effect high-speed control, and this, in turn, entails a relatively long time to converge the control. Thus, addressed is a shortcoming that the impaired characteristics persist for a long time.
Further, the technique disclosed in the third prior art device, in which the distortion detection loop (the error detection loop) is controlled so as to minimize the pilot signal in the output of the auxiliary amplifier by injecting the pilot signal instead of a carrier in order to automatically control the error detection loop, causes the pilot signal to leak outside in theory, and hence addresses a shortcoming that means for preventing the leakage of the pilot signal needs to be provided separately.
Further, the technique disclosed in the fourth prior art device, in which control is effected in accordance with the temperatures measured in the vicinity of the amplifiers by locating the temperature sensors in the vicinity of the amplifiers, respectively, imposes difficulties in preparing a table of temperatures versus control values. When the temperature characteristics are obtained for each amplifier, a highly accurate table can be prepared. However, measuring of the temperature characteristics for each amplifier as a product enormously increases the production cost, and hence is not preferable. On the other hand, when the temperature characteristics of amplifiers are assumed to be consistent, production-related problems can be avoided. But, when there are fluctuations between amplifiers, the characteristics of the amplifiers deteriorate. Further, in this technique in which the temperature characteristics of each amplifier does not change during the operation, long term fluctuations of the amplifier characteristics cannot be well taken care of.